Bypassing a device in a scan chain

ABSTRACT

Methods and systems for testing devices in a scan chain are described. A first device for test and a second device for test are coupled in the scan chain. A signal selector is coupled between the first and second devices. The signal selector selects between an output signal that is output from the first device and a bypass signal that has bypassed the first device.

TECHNICAL FIELD

[0001] Embodiments of the present invention relate to testing of devicesin a scan chain. More specifically, embodiments of the present inventionrelate to boundary scan testing.

BACKGROUND ART

[0002] With advances in design, device components are being placedcloser together in smaller-sized packages. As a result, traditionaltechniques for testing devices can be difficult to implement.

[0003] The Joint Action Test Group (JTAG) was formed by manufacturers toaddress testing problems associated with smaller devices. The findingsand recommendations of JTAG were used as the basis for the Institute ofElectrical and Electronic Engineers (IEEE) Standard 1149.1, “StandardTest Access Port and Boundary-Scan Architecture.” This standard iscommonly referred to as JTAG. Boundary scan testing as defined by JTAGprovides the ability to set and read values on device pins withoutdirect physical access. According to JTAG, there are a number ofregisters associated with boundary scan testing. One of these registersis the Bypass register, which is a single-bit register that passesinformation from the “test data in” (TDI) to the “test data out” (TDO)pin. Devices under test can be can be coupled in series in what isreferred to as a “scan chain.” The Bypass register allows the testing ofother devices in a scan chain without unnecessary overhead.

[0004] However, testing of devices coupled in a scan chain can beproblematic when one of the devices is powered off or is inoperable forsome reason. When this occurs, there is a break in the scan chain andtest signals cannot be passed down the chain. Therefore, withconventional scan chain testing, the chain needs to be intact, with alldevices in the chain powered on and operable. Conventionally, thetesting is halted until the scan chain is again intact; that is, testingis not performed until the device that is powered off or inoperable iseither powered on, returned to an operable state, or physically removedfrom the chain. As a consequence, testing is made more difficult, andthe time needed for testing is increased. Accordingly, a system and/ormethod that can allow scan chain testing when one or more of the devicesin the scan chain are powered off or otherwise inoperable would bevaluable.

DISCLOSURE OF THE INVENTION

[0005] Embodiments of the present invention pertain to methods andsystems for testing devices in a scan chain. In one embodiment, a firstdevice for test and a second device for test are coupled in the scanchain. A signal selector is coupled between the first and seconddevices. The signal selector selects between an output signal that isoutput from the first device when the first device is powered on,operable and included in the scan chain, and a bypass signal that hasbypassed the first device when the first device is powered off, deemedinoperable, or isolated from the scan chain.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The accompanying drawings, which are incorporated in and form apart of this specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention:

[0007]FIG. 1 is a block diagram of a boundary scan compliant deviceaccording to one embodiment of the present invention.

[0008]FIG. 2A is a block diagram of a system for testing devicesaccording to one embodiment of the present invention.

[0009]FIG. 2B is a block diagram of a system for testing devicesaccording to another embodiment of the present invention.

[0010]FIG. 3 is a flowchart of a method for testing devices according toone embodiment of the present invention.

[0011] The drawings referred to in this description should not beunderstood as being drawn to scale except if specifically noted.

BEST MODE FOR CARRYING OUT THE INVENTION

[0012] Reference will now be made in detail to various embodiments ofthe invention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction withthese embodiments, it will be understood that they are not intended tolimit the invention to these embodiments. On the contrary, the inventionis intended to cover alternatives, modifications and equivalents, whichmay be included within the spirit and scope of the invention as definedby the appended claims. Furthermore, in the following description of thepresent invention, numerous specific details are set forth in order toprovide a thorough understanding of the present invention. In otherinstances, well-known methods, procedures, components, and circuits havenot been described in detail as not to unnecessarily obscure aspects ofthe present invention.

[0013]FIG. 1 is a block diagram of a boundary scan compliant device 1according to one embodiment of the present invention. Specifically,device 1 is substantially compliant with the Institute of Electrical andElectronic Engineers (IEEE) Standard 1149.1, “Standard Test Access Portand Boundary-Scan Architecture.” Device 1 can also be said to besubstantially compliant with the recommendations of the Joint TestAction Group (JTAG). Accordingly, in the present embodiment, device 1can utilize boundary scan testing.

[0014] Boundary scan compliant, JTAG compliant devices, and boundaryscan testing are known in the art; nevertheless, a brief description isincluded herein. Boundary scan testing provides the capability to setand/or read values on pins (exemplified by pin 10) without directphysical access. Signals between core logic 16 of device 1 and the pins(e.g., pin 10) are intercepted by boundary scan cells (exemplified byboundary scan cell 12). During normal operation of device 1, theboundary scan cells are invisible. However, during testing, the boundaryscan cells can be used to set and/or read values. These values can bethe values of the pins or the values of the core logic 16, depending onthe test mode.

[0015] The “test data in” (TDI) of FIG. 1 represents data shifted in thetest or programming logic of device 1. The “test data out” (TDO) of FIG.1 represents the data shifted out of the test or programming logic ofdevice 1.

[0016] Registers 14 represent the collective set of registers used by aboundary scan compliant device. The registers 14 can include dataregisters and instruction registers. The registers 14 and the boundaryscan cells (e.g., boundary scan cell 12) are part of the scan path.Signals between core logic 16 and the pins of device 1 are interceptedby the scan path during testing.

[0017]FIG. 2A is a block diagram of a system 20 for testing devicesaccording to one embodiment of the present invention. In the presentembodiment, system 20 includes device 1 coupled to device 2 in a scanchain. Device 1 and device 2 are boundary scan compliant devices.Devices 1 and 2 can be, but are not limited to, integrated circuits, forexample.

[0018] Coupled between device 1 and device 2 is a signal selector 24. Inone embodiment, selector 24 is a multiplexer.

[0019] System 20 can include any number of devices coupled in a scanchain. Consecutive devices in the scan chain may or may not be coupledto an intervening signal selector. That is, a signal selector may or maynot be coupled between two devices that are coupled in series in thescan chain. As will be seen, a signal selector can be used to selectbetween an output signal and a bypass signal of an upstream device,depending on whether or not the device is powered on or powered off, forexample. If the device is always powered on, then a signal selectordownstream of that device may not be needed and hence can be eliminated.In other words, referring to FIG. 2A, if for example device 1 is alwayspowered on, then selector 24 may be eliminated from the design of system20.

[0020] For brevity, FIG. 2A is discussed for device 1. The discussion ofFIG. 2A can be readily extended to device 2 as well as to other devicesthat may be present in system 20.

[0021] Coupled to device 1 is an input line 26 and an output line 28.Output line 28 is coupled to the selector 24. Bypass line 30 couplesinput line 26 to selector 24, bypassing device 1.

[0022] In the present embodiment, a power rail 22 for device 1 is alsocoupled to device 1 and selector 24. In this embodiment, device 1 ispowered using power rail 22, while power rail 22 is coupled to aselection pin 25 of selector 24. Selector 24 receives power from standbypower rail 32 or from some other stable source of power. Device 2receives power from power rail 36.

[0023] Thus, in the present embodiment, device 1 and selector 24 areboth coupled to the same power rail. Accordingly, selector 24 canreadily detect whether device 1 is powered on or powered off.Significantly, because device 1 and selector 24 are coupled to the samepower rail, selector 24 can be implemented without additional controllogic.

[0024] System 20 is now described in operation for device 1. An inputsignal travels over input line 26 to device 1. If device 1 is poweredon, the input signal is also delivered to selector 24 via output line28. The input signal also travels over bypass line 30 to selector 24.Selector 24 will always receive the input signal via bypass line 30,although selector 24 may not always select the signal delivered overbypass line 30, as described below.

[0025] If device 1 is powered on, then power rail 22 is high, and thiscauses selector 24 to select signals from output line 28. If device 1 ispowered off, then power rail 22 is low, and this causes selector 24 toselect signals from bypass line 30. The selected signal (selector output34) is forwarded to the next device in the scan chain (e.g., device 2).

[0026] Note that should device 1 become inoperable for some reason, thenpower to that device can be turned off. As a result, selector 24 willselect signals from bypass line 30.

[0027] Thus, according to the embodiments of the present invention, thescan chain remains intact and testing can proceed when device 1 ispowered off or is otherwise inoperable, without having to physicallyremove the device from the scan chain.

[0028] Note also that device 1 can be selectively removed from the scanchain by powering it off. There may be reasons why it is desirable toisolate a device from the scan chain, and the features of the presentinvention facilitate that effort, without having to physically removethe device from the scan chain.

[0029]FIG. 2B is a block diagram of a system for testing devicesaccording to another embodiment of the present invention. One differencebetween the embodiment of FIG. 2B and the embodiment of FIG. 2A is theuse of a control signal 35 provided over a control line coupled toselector 24. Under control of the control signal 35, selector 24 selectseither the output of device 1 (the signal delivered over output line 28)or the bypass signal (the signal delivered over bypass line 30). Controlsignal 35 can be set in a number of different ways. Control signal 35can be set independent of whether or not device 1 is powered on. Thus,even if device 1 is powered on and operable, device 1 can be isolatedfrom the scan chain by selecting the bypass signal in response to thecontrol signal 35.

[0030]FIG. 3 is a flowchart 300 of a method for testing devicesaccording to one embodiment of the present invention. Although specificsteps are disclosed in flowchart 300, such steps are exemplary. That is,embodiments of the present invention are well suited to performingvarious other steps or variations of the steps recited in flowchart 300.It is appreciated that the steps in flowchart 300 may be performed in anorder different than presented.

[0031] In step 310, a signal selector (e.g., selector 24 of FIGS. 2A and2B) selects a signal that has bypassed a device that is upstream of thesignal selector (e.g., device 1 of FIGS. 2A and 2B). Specifically, thesignal selector selects the signal delivered via bypass line 30 of FIGS.2A and 2B. The upstream device may be powered off or otherwiseinoperable, or the device may be powered on and operable. In eithercase, the upstream device is isolated from the scan chain whilemaintaining the integrity of the scan chain.

[0032] In step 320 of FIG. 3, the signal selector can instead select asignal that is an output of the upstream device. For example, when theupstream device is powered on or otherwise is operable, then the signalselector selects the signal delivered via output line 28 of FIGS. 2A and2B.

[0033] In step 330 of FIG. 3, the selected signal is forwarded to thenext device in the scan chain (e.g., device 2 of FIGS. 2A and 2B).

[0034] In summary, embodiments of the present invention provide methodsand systems that allow scan chain testing when one or more of thedevices in the scan chain are powered off or otherwise inoperable, or isselectively isolated from the scan chain. As a result, scan chaintesting is facilitated and can be accomplished more quickly.

[0035] Although described for multiple devices coupled in a scan chain,the features of the present invention, in its various embodiments, canalso be used to perform similar testing of components within a singledevice. That is, a device may include a number of internal devices thatare coupled in series, and a signal selector can be situated between theinternal devices and used during testing in a manner similar to thatdescribed above.

[0036] Embodiments of the present invention are thus described. Whilethe present invention has been described in particular embodiments, itshould be appreciated that the present invention should not be construedas limited by such embodiments, but rather construed according to thefollowing claims.

What is claimed is:
 1. A system for testing devices, said systemcomprising: a first device for test; a second device for test coupled tosaid first device in a scan chain; and a signal selector coupled betweensaid first and second devices, said signal selector for selectingbetween an output signal that is output from said first device and abypass signal that has bypassed said first device.
 2. The system ofclaim 1 wherein said signal selector is coupled to a first power railthat is also coupled to said first device.
 3. The system of claim 2wherein said signal selector selects said bypass signal when said firstpower rail is low and said output signal when said first power rail ishigh.
 4. The system of claim 2 wherein said signal selector is alsocoupled to a standby power rail that powers said signal selector.
 5. Thesystem of claim 1 wherein said signal selector selects between saidoutput signal and said bypass signal in response to a control signal. 6.The system of claim 1 wherein said second device is coupled to a secondpower rail.
 7. The system of claim 1 wherein said signal selector is amultiplexer.
 8. The system of claim 1 wherein said first and seconddevices are boundary scan compliant devices.
 9. A method of testingdevices in a scan chain, said method comprising: selecting a bypasssignal that has bypassed a first device; selecting an output signalinstead of said bypass signal, said output signal being output from saidfirst device; and forwarding a selected signal to a second device insaid scan chain.
 10. The method of claim 9 wherein said selectingbetween said bypass signal and said output signal is performed by asignal selector that is coupled to a first power rail that is alsocoupled to said first device.
 11. The method of claim 10 wherein saidsignal selector selects said bypass signal when said first power rail islow and said output signal when said first power rail is high.
 12. Themethod of claim 10 wherein said signal selector is also coupled to astandby power rail.
 13. The method of claim 9 wherein said selectingbetween said bypass signal and said output signal is performed by asignal selector in response to a control signal received by said signalselector.
 14. The method of claim 9 wherein said first and seconddevices are boundary scan compliant devices.
 15. A system for testingdevices, said system comprising: a first device for test coupled in ascan chain; an input line coupled to said first device; an output linecoupled to said first device; a bypass line coupled to said input lineupstream of said first device; and a signal selector coupled to saidoutput line and to said bypass line, wherein said signal selectorselects from an output signal on said output line and an input signaldelivered to said signal selector via said bypass line.
 16. The systemof claim 15 wherein said signal selector is coupled to a first powerrail that is also coupled to said first device.
 17. The system of claim16 wherein said signal selector selects said input signal when saidfirst power rail is low and said output signal when said first powerrail is high.
 18. The system of claim 16 wherein said signal selector isalso coupled to a standby power rail that powers said signal selector.19. The system of claim 15 wherein said signal selector is coupled to acontrol line that provides a control signal, wherein said signalselector selects between said output signal and said input signal inresponse to said control signal.
 20. The system of claim 15 furthercomprising: a second device for test coupled to said signal selector.21. The system of claim 20 wherein said first device is coupled to onepower rail and wherein said second device is coupled to a differentpower rail.
 22. The system of claim 15 wherein said signal selector is amultiplexer.
 23. The system of claim 15 wherein said first device isboundary scan compliant.